Method for reducing the thermal load on an automatic transmission of a motor vehicle in emergency operating mode

ABSTRACT

The invention relates to an automatic transmission of a motor vehicle which is controlled by an electronic transmission control system which continuously exchanges signals with a digital engine electronics system of an internal combustion engine. To reduce the thermal load on the automatic transmission in emergency operating mode, a transmission-independent program module (M 1 ) of the digital engine electronics system sets an admissible maximum vehicle speed for the emergency operating mode (v_max_not) or an admissible maximum engine speed for the emergency operating mode (n_mot_max_not).

FIELD OF THE INVENTION

The invention relates to a method for reducing the thermal load of anautomatic transmission for a motor vehicle in an emergency operatingmode.

BACKGROUND OF THE INVENTION

The utilization of the intelligence in electronics finds its expressionin motor vehicles having modern automatic transmissions in the use of anelectronic transmission control (EGS), which to a great extent, meetsthe known criteria to be relevant for the customers, such as drivingcomfort and drivability, and also relevant to security. For theselection of gear, according to situations, the electronic transmissioncontrol steadily communicates with other control units and computers ofdifferent aggregates via a CAN (controller area network) data bus.

For the case of a defect of the electronic transmission control,emergency operating programs have often been provided which are designedso that in an emergency operating mode a vehicle can be operated in thewidest possible driving range. Depending on the emergency running, thecooling of the transmission is problematic in such an emergencyoperating mode, since the heat accumulation in the emergency operatingmode of the transmission must be eliminated via the cooling system ofthe vehicle and, in case of complete failure of the electronictransmission control, the transmission has no positive way of its own toact upon the thermal load.

This particularly occurs in CVT (Continuously Variable Transmission)automatic transmissions which have a variator for continuous adjustmentof a reduction ratio between a shortest possible ratio (LOW) and alongest possible ratio (overdrive, OD).

Such a CVT transmission with a first cone pulley pair upon an inputshaft as primary pulley set and with a second cone pulley pair upon anoutput shaft as secondary pulley set has been described in “ATZAutomobiltechnische Zeitschrift” 96 (1994). Each cone pulley pairconsists of a first pulley axially stationary and a second pulleyaxially movable which are designated as a primary pulley and a secondarypulley whereby they belong to the primary or secondary pulley sets,respectively. Between the cone pulley pairs rotates atorque-transmitting element, for example, a linked band, wound aroundthe cone pulley pairs. To adjust the primary pulley and the secondarypulley, they are loaded with a pressure medium from a pressure source,the pressure being applied, respectively, to the adjusting chambers ofthe primary pulley and secondary pulley with the aid of an electronictransmission control and an electrohydraulic pressure-supply device.

DE 44 36 506 has disclosed a device for control of a CVT transmission inwhich an electronic control unit determines, via electromagneticactuators and hydraulic valves, the pressure level of the adjustingchambers of primary pulley and secondary pulley which are notdynamically balanced.

For a failure of the electronic control unit, an emergency operatingdevice is provided which has two pressure-regulating valves, twopressure-reducing valves and at least one emergency valve, wherein aprimary valve and a secondary valve adjust a constant static pressureratio or power ratio between primary pulley and secondary pulley inconstant static secondary pressure level.

By the amount that the static power ratio and the dynamic forces on thepulleys is not dynamically pressure balanced, the magnitude of the ratiochange of the CVT transmission from a normal operation to the emergencyoperating mode can be established. The emergency operating mode is atthe same time described as a state of the CVT transmission in which arotational speed regulation and a contact regulation of the secondarypulley are interrupted.

In this known device, the ratio of the CVT transmission varies accordingto the generated torque of an internal combustion engine provided asinput unit, wherein the variable ratio change in the emergency operatingmode makes a reliable start on the hill and improved driveability athigh top speed possible.

As input signals abut on an electronic control unit, e.g. the signal ofa load position of the input unit like the throttle valve position incase of an internal combustion engine, the rotational speed of thetransmission input shaft, the rotational speed of the output shaft, orthe temperature of the pressure medium. From the input signal, anoperating point is determined and the appertaining rotational speed ofthe transmission input shaft or the ratio of the CVT transmissionadjusted. When an error occurs, the CVT transmission changes over to theemergency operating mode in which no rotational speed regulation and nocontact pressure regulation of the secondary pulley take place any more.

Even though this known solution offers a wide possible adjustment rangeof the variator, it is problematic that the contact pressure for thecorresponding pulleys must be designed according to the extreme pointsof the driveability range. One of the points is the guaranty of themaximum starting power in the lowest gear, i.e. in or near the LOWratio. For the variator capacity of transmission, a higher contactpressure on the secondary pulley is required at high ratio and highinput torque. As result of the constant static pressure ratio betweenprimary pulley and secondary pulley, the static force adjusted on thesecondary pulley, whose magnitude depends on the maximum input torque tobe transmitted, is constant in the whole torque range.

In wide driving ranges, the consequence of this is an undesiredoverpressure, which disadvantageously leads to a high operatingtemperature in the variator with increased loss of effectiveness andwear of the variator system.

To reduce an inadmissible thermal load of the input engine of a vehicle,in WO 93/13340 has been disclosed a diagnosis module designed asself-sufficient control module detects erroneous conditions of the drivetrain via defined limiting value observations of sensor signals or inputsignals delivered by other control units of the drive train. Dependingon faulty gravity, the diagnosis module more or less limits the maximumadmissible vehicle speed. Combined with a transmission control, WO93/13340 proposes to limit the vehicle speed by the gear selection ofthe transmission, the relevant gear of the transmission control beingpreset by the diagnosis module according to the maximum admissiblevehicle speed, which presupposes a fully operable transmission control.

SUMMARY OF THE INVENTION

The problem on which this invention is based is to make a methodavailable for reducing the thermal load of an automatic transmission fora motor vehicle in an emergency operating mode in which method thethermal and mechanical damages on the automatic transmission areprevented due to heat action during the emergency operating mode.

With the aid of the inventive method, a maximum vehicle speed v_max_notor maximum engine rotational speed n_mot_max_not is preset by thedigital motor electronic system in the emergency operating mode, it ispossible to prevent, by a simple limitation of the operating range, athermal overstress and a heat-determined wear of components of theautomatic transmission, the same as a thermal aging of the transmissionoil.

In the inventive method, the necessary protective functions for thetransmission are perceived by the engine electronic system whereby amore effective overheat protection is still ensured when the electronictransmission control has entirely failed, has been electronicallydisconnected, or has been electrically separated from the vehiclecircuit electronics.

The limitation of the vehicle speed or the limitation of the enginerotational speed, which likewise entails a limitation of the highestspeed of the vehicle, by a program module in the engine electronicssystem represents a simple, but effective step, both in regard to anelectrohydraulically controlled stepped automatic transmission ofconventional type and also in regard to a CVT transmission.

The use of the inventive method is of special advantage in CVTtransmissions, since with only a slight limitation of the operatingcondition is prevented here; the entrance of heat in the variator due toovercontact with the attendant consequences of wear.

In addition by omitting extreme angular points of the driveability inthe emergency operating mode, the dimensions of the transmission oilcooler can be kept small.

BRIEF DESCRIPTION OF THE DRAWING(S)

Other advantages and developments of the invention form the embodimentfundamentally described herebelow with reference to the drawing.

The drawing shows a flow chart in basic characteristics of an inventivemethod for reducing the thermal load of an automatic transmission inemergency operating mode.

DETAILED DESCRIPTION OF THE INVENTION

To apply the method, in a digital engine electronics system of aninternal combustion engine (not shown in detail) which exchanges signalsvia a CAN (controller area network) data bus with an electronictransmission control device, a program module M1 is provided which isindependent of the transmission control.

In the program module M1 is first constantly tested in a first functionF1 whether a CAN signal indicative of a normal operation of theelectronic transmission control device enters in the digital engineelectronics system. When this is the case, the digital engineelectronics system issues in a subsequent function F2 as admissiblevehicle speed v_zul a highest speed provided for the vehicle in thenormal operation v_max.

If due to the failure of the CAN signal that dominates the normaloperation, it is detected that the transmission is in emergencyoperating mode, in a function F3 the digital engine electronics systemlimits the admissible vehicle speed v_zul to a maximum vehicle speed foran emergency operating mode v_max_not such as 130 km/h.

In the emergency operating mode, it is tested in one otherdifferentiation function F4 whether an actual vehicle speed v_actual islower than or equal to the maximum vehicle speed for the emergencyoperating mode v_max_not. If this is the case, the beginning of theprogram module M1 is branched back to.

However, when the actual vehicle speed v-actual is higher than thepreset admissible maximum vehicle speed for emergency operating modev_max_not, a transition function F5, likewise implemented in the digitalengine electronics system, is started which ensures a reliabletransition from the actual vehicle speed v_actual to the admissiblemaximum vehicle speed for the emergency operating mode v_max_not.

The time-dependent transition function F5 reduces the actual vehiclespeed v_actual ramp-like, that is slowly, until the admissible maximumvehicle speed for the emergency operating mode v_max_not is reached.Thereby an abrupt change of speed with accompanying losses in drivingcomfort and vehicle instabilities critical to safety is prevented.

In another embodiment (not shown) to the ramp-like transition functionF5 can be connected in series a discrete time step during which theactual vehicle speed v_actual is accepted without engagement.

The actual vehicle speed v_actual is reduced during the course of thetransition function F5 by a slow removal of a fuel injection amount ofthe internal combustion engine or by a slow removal of a throttle valveangle of the internal combustion engine operatively connected with anelectronic accelerator pedal.

As input signal for rating or processing, the actual vehicle speedv_actual serve speed portional variables like a wheel rotational speedrecorded via a sensor of an antiblocking system.

In the instant embodiment, the method for reducing the thermal load ofthe automatic transmission in emergency operating mode is applied to aCVT automatic transmission which has a variator with a primary pulleyset, a secondary pulley set and a pushing linked band wound around themas torque-transmitting element. In the emergency operating mode, acontact pressure regulation of the secondary pulley set is interruptedand a constant static pressure ratio is adjusted between the primarypulley set and the secondary pulley set.

By limiting the admissible vehicle speed v_zul in the emergencyoperating mode of the transmission, the operating range of the vehicleis purposefully limited, the operating modes of the CVT automatictransmission being reduced with higher overpressing on the variatorwithout clearly limiting the operating readiness of the vehicle.

Alternatively to the method shown in the drawing, it is obviouslypossible instead of the limitation of the admissible vehicle speed v_zulin the emergency operating mode of the transmission, to carry out alimitation of the admissible engine rotational speed, e.g. to a maximumrotational speed of 4000 rpm, the vehicle availability being, of course,further reduced. The method itself develops similarly to the methodshown.

In a CVT automatic transmission with a starting clutch on the outputside, as consequence of the direct coupling of the engine with thevariator and the reduction ratio that appears according to the enginetorque in the absence of positive engagement, the variator can beadjusted by gas supply according to an overdrive ratio. The secondarypulley of the variator must be protected against overspeed.

In addition as one other method step not shown, the actual enginerotational speed n_mot_actual is immediately limited to the admissiblemaximum engine rotational speed n_mot_max_not when the emergencyoperating mode of the automatic transmission is detected in a selectorlever position not requiring positive engagement such as “P” and “N”.

When the actual engine rotational speed n_mot_actual is of course higherthan the admissible maximum engine rotational speed n_mot_max_not andthe emergency operating mode of the automatic transmission is detectedin a selector lever position requiring positive engagement, the actualengine rotational speed n_mot_actual is changed over in a ramp-liketransition function to the admissible maximum engine rotational speedn_mot_max_not.

In another embodiment, the inventive method an also be provided incombination with a torque converter automatic transmission in which alock-up clutch is opened in the emergency operating mode. The limitationof the vehicle speed or of the engine rotational speed advantageouslyreduces the possible great charge of heat in the automatic transmissionresulting from the torque converter power loss.

Finally, to improve the cooling of the transmission, in otherembodiments an engine ventilator is started which is operated at anadaptable coolant temperature lowered, e.g. by 20 K C_Luefter_ein_not.Alternatively to this, the engine ventilator obviously can also runpermanently.

Reference numerals F1 differentiation function F2 processing function F3processing function F4 differentiation function F5 transition functionM1 program mode

v_actual actual vehicle speed

v_maxadmissible maximum vehicle speed in normal operation

v_max_not admissible maximum vehicle speed in emergency operating mode

v_zul admissible vehicle speed

What is claimed is:
 1. A method for reducing the thermal load of anautomatic transmission for a motor vehicle during operation in anemergency operating mode wherein the automatic transmission iscontrolled by an electronic transmission control device which exchangessignals with a digital engine electronics system of an internalcombustion engine, and the electronic transmission control device beingunable to function during the emergency operating mode, the methodcomprising the steps of: (1) detecting when the vehicle is operating inthe emergency operating mode and when the vehicle is operating in anormal mode; (2) when the vehicle is operating in the normal mode,determining a highest speed of operation for the vehicle as anadmissible speed of vehicle operation; (3) when the vehicle is operatingduring the emergency operating mode, determining a preset admissiblevehicle emergency mode speed as a preset maximum vehicle speed foroperating during the emergency mode; (4) determining if an actualvehicle speed of the vehicle is higher than the preset maximum vehiclespeed for operating in the emergency operating mode; and (5) when theactual vehicle speed is determined as being higher than the presetmaximum vehicle speed for operating in the emergency operating mode,reducing the actual vehicle speed to the preset maximum vehicle speed.2. The method according to claim 1, further comprising the step of,during operating in the emergency operating mode, presetting, via atransmission-independent program module (M1) of the digital engineelectronics system, one of an admissible maximum vehicle speed for theemergency operating mode (v_max_not) and an admissible maximum enginerotational speed for the emergency operating mode (n_mot_max_not) as theadmissible maximum vehicle speed for the emergency operating mode. 3.The method according to claim 2, further comprising the step ofpresetting one of the admissible maximum vehicle speed for the emergencymode (v_max_not) and the admissible maximum engine rotational speed forthe emergency operating mode (n_mot_max_not) when the digital engineelectronics system detects the emergency operating mode of the automatictransmission resulting from a failure of a signal of the electronictransmission control device indicative of a normal operation.
 4. Themethod according to claim 3, further comprising the step of entering asignal into the digital engine electronics system, indicative of thenormal operation of the electronic transmission control device, as a CANsignal.
 5. The method according to claim 2, further comprising the stepof, when one of the actual vehicle speed (v_actual) is higher than oneof the admissible maximum vehicle speed for the emergency operating mode(v_max_not) and the actual engine rotational speed (n_mot_actual) ishigher than the admissible maximum engine rotational speed for theemergency operating mode (n_mot_max_not), starting a transition function(F5) to convert an actual vehicle speed (v_actual) to one of theadmissible maximum vehicle speed for the emergency operating mode(v_max_not) and an actual engine rotational speed (n_mot_actual) to theadmissible maximum engine rotational speed for the emergency operatingmode (n_mot_max_not).
 6. The method according to claim 5, furthercomprising the step of converting the actual vehicle speed (v_actual) tothe preset maximum vehicle speed for operating in the emergencyoperating mode (v_max_not) by one of a modifying of a fuel injectionamount for the internal combustion engine and a modifying of a throttlevalve angle for the internal combustion engine operatively connectedwith an electronic accelerator pedal.
 7. The method according to claim5, further comprising the step of converting, via the transitionfunction (F5), one of the actual vehicle speed (v_actual) and the actualengine rotational speed (n_mot_actual) in a ramp-like manner to one ofthe admissible maximum vehicle speed for the emergency operating mode(v_max_not) and the admissible maximum engine rotational speed for theemergency operating mode (n_mot_max_not).
 8. The method according toclaim 5, further comprising the step of accepting, without engagement,one of the actual vehicle speed (v_actual) and the actual enginerotational speed (n_mot_aktuell) by a series of discrete time steps ofthe transition function (F5).
 9. The method according to claim 2,further comprising the step of controlling, via the electronictransmission control device, a variator of the automatic transmissionwhich has a primary pulley set and a secondary pulley set with atorque-transmitter element wrapped up around the primary and secondarypulley sets and, during operation in the emergency operating mode,interrupting a contact pressure regulation of the secondary pulley setand adjusting a constant static pressure ratio between the primarypulley set and the secondary pulley set.
 10. The method according toclaim 2, wherein the automatic transmission has a starting clutch on theoutput side, and the method further comprising the step of immediatelylimiting the actual engine rotational speed (n_mot_actual) to theadmissible maximum engine rotational speed (n_mot_max_not) when theemergency operating mode of the automatic transmission is detected in aselector lever position not requiring positive engagement.
 11. Themethod according to claim 2, wherein the automatic transmission has astarting clutch on the output side, and the method further comprisingthe step of converting the actual engine rotational speed (n_mot_actual)in a ramp-like transition function to the admissible speed(n_mot_actual) when the actual engine rotational speed is higher thanthe admissible maximum engine rotational speed (n_mot_max_not) and theemergency operating mode of the automatic transmission is detected in aselector lever position requiring positive engagement.
 12. The methodaccording to claim 2, further comprising the step of, when the emergencyoperating mode of the automatic transmission is detected, operating anengine ventilator with an applicable lowered coolant temperature(C_Luefter_ein_not).
 13. A method for reducing the thermal load of anautomatic transmission for a motor vehicle during operation in anemergency operating mode wherein the automatic transmission iscontrolled by an electronic transmission control device which exchangessignals with a digital engine electronics system of an internalcombustion engine, and the electronic transmission control device beingunable to function during the emergency operating mode, the methodcomprising the steps of: (1) detecting when the vehicle is operating inthe emergency operating mode and when the vehicle is operating in anormal mode; (2) when the vehicle is operating during the emergencyoperating mode, determining a preset admissible vehicle emergency modespeed as a preset maximum vehicle speed for operating during theemergency mode; (3) determining if an actual vehicle speed of thevehicle is higher than the preset maximum vehicle speed for operating inthe emergency operating mode; and (4) when the actual vehicle speed isdetermined as being higher than the preset maximum vehicle speed foroperating in the emergency operating mode, reducing the actual vehiclespeed to the preset maximum vehicle speed.